Method for producing highly purified, granular silicium

ABSTRACT

The invention relates to a method for producing granular silicon by thermal decomposition of a gas containing silicon in a fluidized bed, said decomposition occurring in the presence of free-flowing mobile elements. Preferably, said free-flowing mobile elements become devoid of silicon in a separate procedural step, said silicon being deposited during decomposition of gas containing silicon, by reacting with hydrogen halides, halogens, alkyl halogenides, aryl halogenides or combinations of halogen and/or hydrogen halide and/or oxidized mineral acids and/or by thermal treatment of said elements.

[0001] The present invention relates to a method for producing highlypurified granular silicon by decomposition of a of silicic gas and theuse of elemental silicon produced in this way in the photovoltaic areaand in semi-conductor technology.

[0002] Silicic gases as referred to herein are silicon compounds ormixtures of silicon compounds which under the conditions according tothe invention can be decomposed in the gaseous phase depositing silicon.Silicon-free gases in the meaning of this invention are gases which donot contain any silicon compounds.

[0003] For the production of elemental silicon with a purity allowingits being used in the photovoltaic area and/or in semi-conductortechnology, methods of thermal decomposition of volatile siliconcompounds are known. Such thermal decomposition can be carried out, forexample, in fluidized-bed reactors in that small silicon particles areprovided which are then fluidized by an appropriate silicic gas or gasmixture flowing into the reactor, whereby the gases in the gas mixturecan be silicic, but also silicon-free gases. Ideally, the thermaldecomposition of such volatile silicon compounds shall occur exclusivelyon the surface of the small silicon particles are provided. The saidsmall silicon particles provided, hereinafter referred to as nucleusparticles, form a large area for the separation of silicon within thereactor. Particles that have grown to a sufficient size are removed fromthe reactor and new nucleus particles are introduced.

[0004] In addition to the separation of silicon on the particles duringthe decomposition of the gas containing silicon also silicon dust isproduced which is difficult to handle and can be easily contaminated dueto its large surface. Further the produced silicon dust containssignificant amounts of hydrogen obstructing the subsequent melting onprocess. For this reason the formation of dust is undesired. Silicondust in this context refers to silicon particles with a diameter ofparticles of up to approx. 25 μm.

[0005] It is known that in the case of decomposition of gas containingsilicon in fluidized-bed reactors a major part of the dust is formed ina homogeneous reaction. This reaction occurs predominantly in theso-called bubble phase. The silicon formed in such phase bydecomposition of gases containing silicon does not result in anexpansion of provided nucleus particles by separation on said nucleusparticles (Chemical Vapour Deposition CVD), but forms dust that iscarried out from the reactor.

[0006] On principle, so-called bubble breakers are suitable for thereduction of gas bubbles in a fluidized bed thus reducing the bubblephase. Different apparatuses with bubble-breaking function are known,e.g. vertical and horizontal gas distribution plates, geometricalconstructions, such as tube combinations and three-dimensional grids andpackings (tower packing, wires etc.). Such apparatuses have thedisadvantage in fluidized beds and particularly under the aggressivereaction conditions prevailing at thermal decomposition of a gascontaining silicon that the intensive radial and axial mixing can bediminished and that a strong erosion of the built-in elements must beexpected. Such serious mechanical strain limits the number of materialssuitable for the said built-in elements. Apart from this it must beensured, particularly with regard to the high purity required for thedesired use of the produced silicon in the semi-conductor orphotovoltaic area, that the bubble breakers do not carry anycontamination into the silicon. A silicon separation on the bubblebreaker elements leads to internal overgrowing of the reactor. Theconstruction and the scale-up of such bubble breakers are difficult.

[0007] The object of the present invention was to provide a reactionmethod for producing high-purity silicon suitable for use in thephotovoltaic area and in electronics that enables a minimum formation ofdust by minimizing the homogeneous reaction.

[0008] Subject-matter of the invention is a method for producinggranular silicon by thermal decomposition of a gas containing silicon ina fluidized bed, said decomposition occurring in the presence offree-flowing mobile elements.

[0009] The use of free-flowing mobile elements as bubble breakers in afluidized bed is known in principle (E. A. M. Gbordzoe, H. Littman, M.A. Bergougnou, Canadian Journal of Chemical Engineering, 66, 1998,158-162). Such elements are known until now, however, only for use inheterogeneous catalysed gaseous phase reactions. The said mobileelements increase the transport of matter, but flow with the particlesin the fluidized bed and are therefore not exposed to erosion asstrongly as fixed built-ins.

[0010] When such free-flowing mobile elements are used in a method forproducing granular silicon, silicon separates on such elements changingsize and density of the free-flowing mobile elements during reaction.Too big elements with a too high density sink to the ground bysegregation and have no more effect on the bubble size. Furthermore itwas feared that the produced silicon would be contaminated due toimpurities diffusing out of the elements.

[0011] Surprisingly, free-flowing mobile elements are suitablenevertheless for use in the method according to the invention and leadto a clear reduction of the amount of dust formed, without any negativeimpact such as contamination of the product or reduction of theconversion.

[0012] In order to ensure the efficiency of said free-flowing mobileelements also during longer reaction periods, it is preferred to removesilicon in a separate procedural step, said silicon being deposited onsuch free-flowing mobile elements during decomposition of gas containingsilicon, by reacting such silicon with hydrogen halides, halogens, alkylhalogenides, aryl halogenides or combinations of halogen and/or hydrogenhalide and/or oxidized mineral acids.

[0013] The method according to the invention can be carried out indifferent types of reactors, provided that inside the reactor afluidized state of solids develops. Appropriate reactors are alreadyknown. By way of example reactors providing a bubbling or turbulentfluidized bed may be mentioned. The method can be carried out, forexample, continuously or discontinuously. A continuous process ispreferred. The silicon particles formed by separation can be carried outof the reactor continuously or discontinuously.

[0014] Silicic gases to be employed can be silanes, silicon iodides andhalosilanes of chlorine, bromine and iodine. Also mixtures of the namedcompounds can be employed. It is irrelevant whether the silicon compoundis already rendered in gaseous form at room temperature or needs to betransformed into gaseous condition first. The transformation to gaseouscondition can be carried out thermally for example. The use of silanesis preferred. By way of example SiH₄, Si₂H₆, Si₃H₈, Si₄H₁₀ and Si₆H₁₄may be named. Particularly preferred is SiH₄.

[0015] It is possible to carry out the method according to the inventionfor the manufacture of highly purified, granular silicon by adding asilicon-free gas or a mixture of several silicon-free gases. Forexample, the amount of silicon-free gas added can be 0 to 98 volumepercent based on the total amount of gas introduced. It is alsopossible, however, to work without adding any silicon-free gas.

[0016] Suitable silicon-free gases are, for example, noble gases,nitrogen and hydrogen, the silicon-free gases being applicable each gasindividually or any combination of them. Nitrogen and hydrogen arepreferred, particularly preferred is hydrogen.

[0017] Temperature can be varied in the temperature range from 300° C.to 1400° C. The temperature must be high enough, however, to ensure thedecomposition of the silicic gas and must not exceed the meltingtemperature of the produced silicon. In case of SiH₄ being used theadvantageous temperature range is between 500° C. and 1400° C. Adecomposition temperature from 600° C. to 1000° C. is preferred,particularly preferred 620° C. to 800° C. In case of SiI₄ being used therespective range is between 850° C. and 1250° C., for other halosilanesbetween 500° C. and 1400° C.

[0018] The method according to the invention is carried in a fluidizedbed. Solid particles—hereinafter referred to as particles—are providedin the reaction zone of a fluidized-bed reactor. These particles can beintroduced from the exterior continuously. These particles can also beparticles which are generated in the reaction zone. The particles form afixed bed through to which the introduced gas is streamed. The stream-invelocity of the introduced gas is adjusted such that the fixed bed isfluidized and a fluidized bed develops. The respective procedure isgenerally known to the skilled person. The stream-in velocity of theintroduced gas must correspond to at least the loosening velocity (alsoreferred to as minimum fluidizing velocity u_(mf)). Loosening velocityin this case is to be understood as the velocity at which a gas streamsthrough a bed of particles and below which the fixed bed is maintained,i.e. below which the bed particles remain largely fixed. Above thisvelocity the bed starts fluidizing, i.e. the bed particles move andbubbles begin to emerge.

[0019] Preferably the stream-in velocity of the introduced gas is one toten times the loosening velocity, particularly preferred one and a halfto seven times the loosening velocity. Preferably particles of adiameter of 50 to 5000 μm are used.

[0020] The particles used are preferably silicon particles. Preferablysuch silicon particle have a purity corresponding to the one desired forthe produced highly purified, granular silicon. It is also possible,however, to use silicon particles with a certain doping level if dopedmaterial is desired. Also particles not consisting of silicon aresuitable provided they are stable under the said reaction conditions.

[0021] The free-flowing mobile elements used for splitting of bubblesand intensifying the gas/solids contact are preferably elements with adensity between 1.0 g·cm³ and 5 g·cm⁻³, particularly preferred 1.1 to2.6 g·cm³. The density of the free-flowing mobile elements must be atleast the density of the emulsion phase.

[0022] Preferably the free-flowing mobile elements have an averagediameter that is at least one order of magnitude higher than the averagediameter of the particles contained in the fluidized bed.

[0023] The free-flowing mobile elements can have, for example, aspherical, ellipsoide, cylindrical or diskoid, symmetrical orasymmetrical or an irregular exterior form.

[0024] The free-flowing mobile elements can have a massive, porous orhollow interior. When hollow elements are used the density of suchfree-flowing mobile elements can be adjusted by filling with a solid. Incase of a hollow type the elements can be damaged, be filled with bedmaterial and sink, but they are mechanically stabilized by thedeposition of silicon on them. The use of hollow or porous free-flowingmobile elements is preferred.

[0025] Different materials are suitable for such free-flowing mobileelements. It must be ensured, however, that the elements are stable tothe mechanical strain to which they are subjected in the fluidized bed.Suitable materials are, for example, silicon, metallic materials, e.g.special steel, non-metallic materials, ceramic materials or compositematerials.

[0026] Since the free-flowing mobile elements are soon covered by asilicon layer in the reactor, they obtain quickly an abrasion resistancesimilar to the one of the silicon formed, therefore also materials suchas graphite can be used.

[0027] Particularly preferred the elements consist of a material whichdoes not contaminate the silicon. Suitable materials are for examplesilica glass, graphite, silicon carbide or silicon or ceramic orcomposite materials.

[0028] It showed that free-flowing mobile elements consisting ofmaterials of a sufficient purity and a density above the density of theemulsion phase, i.e. the phase in a fluidized bed with a highconcentration of solids, and below the density of the particles providedin the fluidized bed are suitable to reduce the size of the gas bubblesforming in the fluidized bed considerably. This minimizes the reactionzone for the decomposition of silicic gas to silicon dust mainlyoccurring in the bubble volume. Further the splitting of bubbles resultsin silicon particles “raining” through the area of high concentrationonto the dust formed due to the homogeneous decomposition of gascontaining silicon, cementing such dust on the surface of the particles,such process being called “scavenging” in the relevant literature.

[0029] It is preferred to remove silicon in a separate procedural step,said silicon being deposited on such free-flowing mobile elements duringdecomposition of gas containing silicon.

[0030] To remove the silicon epitaxially grown in the reaction thefree-flowing mobile elements can be collect, for example by segregation,on the bottom of the reactor and then removed. The elements can beregenerated externally then by decomposition of the silicon in achemical reaction. Suitable reactions are, for example, reactions withhydrohalogens, e.g. HF, HCI, HBr, HI, or halogens, e.g. F₂, Cl₂, Br₂,I₂. It is also conceivable to react the silicon with a mixture ofhalogen and/or hydrohalogen and/or oxidized mineral acid, e.g. HNO₃.Another suitable way of decomposing silicon is the reaction with alkylhalogenides, e.g. CH₃Cl, or aryl halogenides. Such conversions can becarried out at temperatures between room temperature and 120° C.,depending on the reaction type. An inert gas, e.g. nitrogen or a noblegas, or a mixture of several inert gases, can be added to the reactiongas or the reaction mixture.

[0031] It is also possible to regenerate the free-flowing mobileelements in situ. To this end, it is possible, for example, to collectthe free-flowing mobile elements first by segregation on the bottom ofthe reactor. Subsequently for example hydrohalogen, halogen, alkylhalogenides, aryl halogenides or mixture of halogen and/or hydrohalogenand/or oxidized mineral acid is introduced in the reactor from below.Preferably the feeding rate is adjusted such that the introducedreactants are reacted completely in the area where the free-flowingmobile elements gathered. This prevents an undesired reaction of theproduced silicon granules.

[0032] To remove the silicon epitaxially grown on the free-flowingmobile elements in the reaction the fact can be utilized that, as arule, the silicon deposited on the free-flowing mobile elements has adifferent thermal expansibility factor than the free-flowing mobileelements. So the elements can be heated or cooled, for example, causinga cracking off of silicon due to different thermal expansibilityfactors.

[0033] Of course it is also possible of course to carry out acombination of the measures specified above to remove the siliconepitaxially grown on the free-flowing mobile elements in the reaction.

[0034] A preferred embodiment of the method according to the inventioncomprises a combination of the two specified procedural steps. In afirst step a gas containing silicon is thermally decomposed in afluidized bed in the presence of free-flowing mobile elements. Silicondeposits on said elements. Once an amount of silicon affecting thefunction of said elements has deposited on such elements, the elementsare regenerated in situ or externally by reacting the formed silicon,e.g. by contact with hydrogen halides, halogens, alkyl halogenides oraryl halogenides, or by thermal treatment.

[0035] The silicon produced according to the inventive method isparticularly suitable for use in the photovoltaic area and for themanufacture of electronic components.

[0036] In the following the method according to the invention is beingdiscussed by means of exemplary operating states and illustrated byexamples, without restricting the inventive idea insofar.

[0037] All pressure values specified refer to the absolute pressure. Thepressure values specified are to be understood as the pressureprevailing behind the fluidized bed as seen in flow direction of theintroduced gas mixture, unless otherwise provided.

EXAMPLES Example 1

[0038] Influence of the Presence of Free-Flowing Bubble Breaker Elementson the Pyrolysis of Silane in a Fluidized Bed.

[0039] In a fluidized-bed reactor (diameter=52.4 mm, height with headextended=1600 mm), 890 g of silicon particles with an average diameterof 349 μm were provided. The experiments were carried out at a pressureof 1150 mbar. After start-up and heating of the fluidized bed to atemperature of 680° C. in hydrogen, the silane concentration (SiH₄) atthe entrance of the reactor was adjusted from 0 to 10 volume percentbased on the fluidizing gas hydrogen. The ratio of the gas velocity u ofthe introduced gas to the minimum fluidizing velocity u_(mf) wasu/u_(mf)=5.

[0040] In an experiment without free-flowing mobile elements 207standard litres of silane were decomposed under the specifiedconditions. In a second experiment 41 free-flowing mobile elementsconsisting of quartz (hollow cylinders, length=8 mm, diameter=6-8 mm,density p=1.1-1.3 g·cm⁻³) were added and 201 standard litres of silanewere decomposed. Table 1 specifies a comparison of the reacted materialand selectivity to form dust, wherein the values obtained in theexperiment in the presence of free-flowing mobile elements consisting ofquartz were set 100% and the other values refer to this value. Theamount of reacted material is only slightly higher by 0.4 percent, butthe dust selectivity achieved is 7.9 percent lower. TABLE 1 Standardizedamount Standardized of selectivity to material reacted, % dust, % withquartz elements 100 100 without additional ele- 99.6 107.9 ments

[0041] The example illustrates clearly the positive influence offree-flowing mobile elements on the reduction of dust selectivity. 0.13g silicon had deposited on the 41 quartz elements employed. The smallamount of deposited silicon is to be attributed to the shortexperimental period and the small percentage of the surface of thequartz elements in the silicon surface in the bed.

Example 2

[0042] Reaction of the Formed Silicon with Hydrogen Chloride.

[0043] 18 of the free-flowing mobile quartz elements which were coveredwith a closed silicon layer subsequent to Experiment 1, were separatedfrom the fluidized bed by screening and were introduced in a secondreactor (fixed bed) of quartz where they were exposed to a mixture ofhydrogen chloride and nitrogen in a mol ratio of 0.33:1, 20 minutes at atemperature of 470° C., and 16 minutes at a temperature of 485° C. Afterinerting and cooling the quartz elements were discharged from thereactor. A mass difference compared to fresh spheres could not bedetected in the applied range of measuring accuracy (±10⁻⁵ g).

1. A method for producing granular silicon by thermal decomposition of agas containing silicon in a fluidized bed, characterized in that thesaid decomposition occurs in the presence of free-flowing mobileelements, wherein silicon which has deposited on the free-flowing mobileelements during decomposition is removed in a separate procedural step.2. A method according to claim 1, characterized in that silicon beingdeposited on such free-flowing mobile elements during decomposition ofgas containing silicon is removed in the separate procedural step byreacting such silicon with hydrogen halides, halogens, alkylhalogenides, aryl halogenides or combinations of halogen and/or hydrogenhalide and/or oxidizing mineral acids.
 3. A method according to claim 1,characterized in that silicon being deposited on such free-flowingmobile elements during decomposition of gas containing silicon isremoved in the separate procedural step by heating or cooling of suchfree-flowing mobile elements.
 4. A method according to claim 1,characterized in that the fluidized bed consists of silicon particleswith a diameter between 50 and 5000 μm, through which the introduced gasstreams in a way such that the silicon particles are fluidized and afluidized bed develops.
 5. A method according to at least one of claims1 to 4, characterized in that the silicic gas used is a silane.
 6. Amethod according to at least one of claims 1 to 5, characterized in thatthe silicic gas used is SiH₄ and that the reaction is carried out attemperatures from 500 to 1400° C.
 7. A method according to at least oneof claims 1 to 6, characterized in that the streaming velocity of theintroduced gas adopts values from 1 to 10 in relation to the looseningvelocity.
 8. A method according to at least one of claims 1 to 7,characterized in that the free-flowing mobile elements have a densitybetween 1.0 g·cm⁻³ and 5.0 g·cm⁻³.
 9. A method according to at least oneof claims 1 to 8, characterized in that the free-flowing mobile elementshave a diameter that is at least one order of magnitude higher than theaverage diameter of the particles contained in the fluidized bed.
 10. Amethod according to at least one of claims 1 to 9, characterized in thatthe free-flowing mobile elements have a spherical, ellipsoide,cylindrical or diskoid, symmetrical or asymmetrical, or an irregularexterior form.
 11. A method according to at least one of claims 1 to 10,characterized in that the free-flowing mobile elements have a massive,porous or hollow interior.
 12. A method according to at least one ofclaims 1 to 11, characterized in that the materials used for thefree-flowing mobile elements are silicon, metallic materials,non-metallic materials, ceramic materials or composite materials.
 13. Amethod for producing photovoltaic components, comprising the followingprocedural steps: producing granular silicon by thermal decomposition ofa gas containing silicon in a fluidized bed, with the decompositiontaking place in the presence of free flowing mobile elements; removingsilicon that has deposited on the free flowing mobile elements duringthe decomposition of the gas containing silicon; manufacturingphotovoltaic components from the produced silicon.
 14. A method ofmanufacturing electronic components, comprising the following proceduralsteps: producing granular silicon by thermal decomposition of a gascontaining silicon in a fluidized bed, with the decomposition takingplace in the presence of free flowing mobile elements; removing siliconthat has deposited on the free flowing mobile elements during thedecomposition of the gas containing silicon; manufacturing electroniccomponents from the produced silicon.